1. Field of the Invention
The invention generally relates to the field of electrooptic materials, and more particularly, to an optical waveguide device structure and method for reducing the effects of temperature in waveguide modulators made on ferroelectric crystals, such as lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3).
2. Related Art
Electrooptic waveguide modulators are commonly used in optical communication systems, signal processing, sensors, and the like. These modulators may be optical intensity modulators, switches, phase or frequency shifters, polarization transformers, wavelength filters, and the like. A class of these modulators are made of ferroelectric materials, such as z-cut lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3). An operational shortcoming of these modulators is temperature dependence. (The terms device and modulator will be used interchangeably.)
Changes in temperature may be caused by a change in the ambient environment or, for example, a change in the temperature of the active region of the modulator caused by electrical dissipation of radio frequency drive power in the modulator's electrodes or substrate. Temperature changes may directly affect the modulator, or may affect it through changes in stress and the photoelastic effect. Changes in stress change the index of refraction of the material, and therefore the bias point phase and operation of the modulator.
Additionally, in ferroelectric crystals pyroelectric (e.g., LiNbO.sub.3), the index of refraction may be changed through the pyroelectric effect, which changes the internal electric field acting upon the modulator waveguides.
However, devices based on modal interference between two single-mode waveguides, such as Mach-Zehnder type interferometers or directional couplers, are mainly sensitive only to changes of the differential index between two waveguides. Therefore, it is the symmetry of the device structure and the drive configuration that will greatly effect the manifested thermal sensitivity of the device (see "Sensitivity of RF Drive Power and the Temperature Stability of Mach-Zehnder Modulators," J. J. Veselka et al., Topical Meetings on Integrated Photonics Research, New Orleans, La., Apr. 13-16, 1992, paper TuG4, pp 200-201; and Skeath et al., "Novel Electrostatic Mechanism in the Thermal Instability of Z-Cut LiNbO.sub.3 Interferometers," Applied Physics Lett. 49(19):1221-1223).
On z-cut LiNbO.sub.3, the electric fields induced by the pyroelectric effect are predominantly in the direction perpendicular to the top surface of the device. Thus, bound charges are observed on the top surface (where the electrodes are normally located) as well as on the bottom surface. It is therefore assumed that devices fabricated on z-cut LiNbO.sub.3 will be more sensitive to temperature variations than those fabricated on the x-cut orientation. While it is possible to reduce the effects of these electric fields on the modulation of the light in the waveguides, e.g., by choosing a symmetric electrode structure and using a large ground plane, most modulators still show considerable temperature dependence. Even worse, with Mach-Zehnder modulators, the inventors have observed fast and somewhat erratic changes in the switching curve of the modulator during and after cooling or heating of the entire modulator.
Thus, what is desired is a modulator structure capable of reducing the inherent temperature sensitivity discussed above. Prior methods employ an electrically conductive (semiconductive) layer covering the entire top surface of the device to equalize the distribution of the bound charges on the top of the crystal (see I. Sawaki et al., "Thermally Stabilized Z-Cut Ti:LiNbO.sub.3 Waveguide Switch," Conference on Lasers and Electro-optics, San Francisco, Calif., Jun. 9-13, 1986, paper MF2, pp. 46-47; and M. Seino et at., "20-GHz 3dB-Bandwidth Ti:LiNbO.sub.3 Mach-Zehnder Modulator," European Conference on Optical Communications, Amsterdam, The Netherlands, 1990, paper ThG 1.5). However, these methods do not prevent non-uniform accumulation of electric charges on the back surface of the device, which may lead to asymmetric electric field distributions in the modulator waveguides. Thus, what is needed is a device structure that screens the bound charges on the back surface of the device uniformly.